terraform/gcp: Allow to use preemptible VM instances (#8480)

contrib/terraform/gcp/modules/kubernetes-cluster/variables.tf

@@ -27,6 +27,10 @@ variable "master_sa_scopes" {
   type = list(string)
 }
 
+variable "master_preemptible" {
+  type = bool
+}
+
 variable "worker_sa_email" {
   type = string
 }
@@ -35,6 +39,10 @@ variable "worker_sa_scopes" {
   type = list(string)
 }
 
+variable "worker_preemptible" {
+  type = bool
+}
+
 variable "ssh_pub_key" {}
 
 variable "ssh_whitelist" {